1. Field of the Invention
The present invention relates to a thin film piezoelectric resonator, which can be used as a high frequency filter or an RF oscillator, and a method of manufacturing the resonator.
2. Description of the Related Art
Surface acoustic wave (SAW) devices, which have advantages in their high resonance quality factors and their miniaturization, are rapidly developed to be used in filter applications such as LC or dielectric bandpass filters. The SAW device basically has a piezoelectric substrate of single crystal material, and two interdigital transducers located thereon. The resonance frequency of such a SAW device is in inverse proportion to the pitch of an electrode finger of an interdigital transducer. To handle higher frequency signals, the pitch should be reduced. For example, handling more than 1 GHz frequency signals, currently demanded in mobile telecommunication, requires a less than 1 micrometer pitch of the electrode finger. However, such narrow pitch makes it difficult to accurately process the transducers, and causes current leakage and breaking of wire easily when high-power signals enter the transducers.
Thin film piezoelectric resonators overcome the drawback of the SAW devices to handle higher frequency signals. The thin film piezoelectric resonator basically has a piezoelectric film and two electrodes therebetween. The resonance frequencies are determined mainly by a thickness of the piezoelectric film. For example, a film thickness of one to two micrometers brings into a resonance frequency of 2 GHz, and a film thickness of 0.4 to 0.8 micrometer brings into a resonance frequency of 5 GHz. The thin film piezoelectric resonators also make it possible to handle signals up to tens of GHz. The thin film piezoelectric resonator is called a film bulk acoustic resonator (FBAR) or a bulk acoustic wave (BAW) device.
FIGS. 13A to 13D are sectional views of a conventional thin film piezoelectric resonator, showing manufacturing process thereof. To complete the thin film piezoelectric resonator, a lower electrode layer 804 is first patterned on a silicon substrate 800 (FIG. 13A). Next, a piezoelectric layer 815 is deposited to cover the surfaces of the lower electrode layer 804 and the silicon substrate 800 (FIG. 13B). A piezoelectric thin film 805 is patterned by etching the piezoelectric layer 815 (FIG. 13C). As a result of this patterning, a part of the lower electrode layer 804 is exposed. In other words, one of the sides of the piezoelectric thin film 805 is located on the lower electrode layer 804. The other of the sides of the piezoelectric thin film 805 is located on the silicon substrate 800, and therefore a part of the edge of the lower electrode layer 804 is covered. Next, an upper electrode layer 807 is formed on the piezoelectric thin film 805. The upper electrode layer 807 is patterned, extending from a part of the top face of the piezoelectric thin film 805 to the silicon substrate 800 to cover the other of the sides of the piezoelectric thin film 805. Specifically, the piezoelectric thin film 805 also functions to insulate between the lower electrode layer 804 and the upper electrode layer 807. An electrode pad 808a is formed to be electrically connected to the upper electrode layer 807, and an electrode pad 808b is formed to be electrically connected to the exposed part of the lower electrode layer 804. A part of the back face of the silicon substrate 800 is then etched to form a cavity 810 right under the lower electrode layer 804 (FIG. 13D). In the above manner, the thin film piezoelectric resonator 80 is completed.
However, the lower electrode layer 804 is exposed in gas stream for ashing to apply and remove the photoresist in the patterning. In other words, the photolithography and etching processes are needed only to form the lower electrode layer 804. As a result, the surface of the lower electrode layer 804 maintains neither the flatness nor the cleanliness immediately after the deposition process because of, for example, the adsorption of impurities. In addition, since such photolithography process and etching process are usually performed with equipment separate from that for the deposition process, the silicon substrate 800 and the lower electrode layer 804 are exposed to air during moving between these equipment. This exposure causes the adsorption of impurities such as oxygen to the surface of the lower electrode layer 804, so that the flatness and cleanliness are ruined. The surface of the lower electrode layer with low flatness and low cleanliness becomes a redeposition base (S1 in FIG. 13B), which has an adverse affect on the characteristics of the piezoelectric layer 815 deposited thereon. Hence, for example, the film quality, such as the orientation and the polarity of the piezoelectric layer 815, is degraded. Such low film quality causes low quality of a single thin film piezoelectric resonator and uneven quality between thin film piezoelectric resonators.
Japanese Patent Application Laid-Open (JP-A) No. 2002-76824 suggests a piezoelectric thin film resonator whose lower electrode, a piezoelectric thin film and an upper layer electrode can be formed with the same equipment for example.
In JP-A No. 2002-76824, although the patterned lower electrode is shown in drawings, how to pattern the lower electrode and the piezoelectric thin film in the same equipment is not disclosed.